A. Field of the Invention
This invention relates to a magnetic recording medium, and in particular relates to a magnetic recording medium used in a hard disk drive and similar.
B. Description of the Related Art
The recording capacities of hard disk drives have risen steadily, and the recording densities of magnetic recording media mounted in such drives have also continued to rise without interruption. When raising recording densities, the horizontal magnetic recording method has been adopted in the prior art, but the problem of thermal fluctuations has become a large impediment. Hence, in recent years a new recording method which resolves the problem of thermal fluctuations, that is, the perpendicular magnetic recording method, has been adopted, and media employing this method have been commercialized.
Even so, there has been no break in the momentum toward higher recording densities of magnetic recording media, and recording densities are continuing to rise at a pace of approximately 50% per year. This rise is accomplished not only through a change in recording method as described above, but through unflagging progress in all engineering fields. For example, where HDI (Head-Disk Interface) technology is concerned, head flying heights have been reduced year after year, and the distance between the head and the magnetic layer of the magnetic recording medium (the magnetic spacing) has gradually grown smaller. In order to reduce the magnetic spacing, there have been unceasing efforts to reduce the thickness of the protective layer formed on the magnetic layer.
At present the thickness of protective layers has already been reduced to from 2 to 3 nm, but further reduction of the protective layer thickness is sought. For example, in the briefing paper of the 25th SRC Technology Session (May 27, 2008), guidelines for the design of hard disk drives of the next generation products are described, and a recording density of 1 Tb/in2 and magnetic spacing of 6.5 nm are proposed. Considering the breakdown of the magnetic spacing, the magnetic head protective layer thickness is approximately 2 nm, the gap between the uppermost face of the magnetic head and the surface of the magnetic recording medium (the head flying height) is approximately 2 nm, and the lubrication layer of the magnetic recording medium is approximately 0.5 nm to 1 nm, and so the thickness of the protective layer of the magnetic recording medium must be 2 nm or less.
On the other hand, the protective layer, as its name implies, serves to protect the magnetic layer made of a metal material so that is does not react with water in the atmosphere or corrosive gas and corrode. Magnetic recording media having protective layers provided with corrosion resistance and other characteristics are, for example, disclosed in Japanese Patent Application Laid-open No. S61-222024, Japanese Patent Application Laid-open No. H8-167138 and Japanese Patent Application Laid-open No. H9-237415.
In Japanese Patent Application Laid-open No. S61-222024, Japanese Patent Application Laid-open No. H8-167138 and Japanese Patent Application Laid-open No. H9-237415, attempts are made to improve the quality of magnetic recording media by focusing on the contact angle of water dripped onto the surface of the protective layer, setting the contact angle within a prescribed range. It is proposed that the contact angle be set to 75° or greater, to 60° or greater, and to 80° or greater in Japanese Patent Application Laid-open No. S61-222024, Japanese Patent Application Laid-open No. H8-167138 and Japanese Patent Application Laid-open No. H9-237415, respectively.
However, in all of Japanese Patent Application Laid-open No. S61-222024, Japanese Patent Application Laid-open No. H8-167138 and Japanese Patent Application Laid-open No. H9-237415 the thickness of the protective layer is 10 nm or greater, and demands for reduced thickness of the protective layer are not adequately addressed.
On the other hand, if the protective layer is made thinner, coverage of the magnetic layer is reduced, and hence there is a tendency for the corrosion resistance of the magnetic recording medium to be degraded.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.